Parallel RF Excitation Pulse Design for MRI B1+ Inhomogeneity
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Solution Overview
Problem
Current MRI systems face challenges in producing RF excitation pulses that simultaneously achieve desired spatial extent and spectral bandwidth, especially at high B0 field strengths, which is necessary for applications requiring B1+ mitigation over a large spectral bandwidth and spatially selective excitation.
Innovation Solution
A method using parallel transmission techniques to design RF excitation pulses that prescribe both spatial and spectral excitation patterns, where gradient waveforms are determined based on estimated B1+ maps to deposit RF energy in specific k-space locations, producing the desired transverse magnetization patterns.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional single-channel RF excitation is used, then device complexity is low, but the ability to achieve spatially selective excitation with spectral bandwidth control is limited
Solution Approach 1:
The patent segments the RF transmission system into multiple independent channels, each capable of transmitting RF pulses with different spatial and spectral characteristics. This segmentation enables the system to achieve complex spatial-spectral excitation patterns that cannot be obtained with a single channel, while managing complexity through modular channel design
Solution Approach 2:
The patent extends the excitation control from traditional single-dimensional spatial selection to a multi-dimensional space that includes both spatial location and spectral frequency. By adding the spectral dimension to spatial excitation control, the system achieves comprehensive spatial-spectral selectivity for mitigating B1+ inhomogeneity effects across different frequency bands
2Manufacturing precision
If high B0 field strength is used, then spectral bandwidth is increased, but B1+ inhomogeneity effects are exacerbated
Solution Approach 1:
The patent applies local quality by tailoring the RF excitation characteristics specifically to different spatial locations and spectral frequencies. Each region of interest receives customized excitation parameters that compensate for local B1+ inhomogeneity, ensuring uniform excitation quality across the entire spectral bandwidth at high B0 field strengths
Solution Approach 2:
The patent dynamically adjusts RF transmission parameters including amplitude, phase, and frequency across multiple channels to compensate for B1+ inhomogeneity. By changing these parameters based on measured or simulated B1+ distributions, the system maintains reliable excitation uniformity while operating at high B0 field strengths that provide increased spectral bandwidth
3Manufacturing precision
If parallel transmission techniques are used, then spatial-spectral excitation control is improved, but device complexity increases
Solution Approach 1:
The patent optimizes RF transmission parameters including amplitude, phase, and timing across multiple parallel channels to achieve precise spatial-spectral excitation patterns. By carefully controlling these parameters, the system achieves high excitation precision while managing the inherent complexity of parallel transmission through efficient parameter optimization
Solution Approach 2:
The patent incorporates feedback mechanisms where B1+ maps are measured or simulated and used to adjust the RF transmission parameters. This feedback loop enables the system to compensate for hardware variations and achieve consistent excitation precision, making the parallel transmission system more manageable despite its increased complexity
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables reliable spatial-spectral RF excitation patterns with reduced B1+ inhomogeneity effects, allowing for efficient MRI operations at high B0 strengths with controlled excitation across a specified spectral bandwidth and spatial range.
Implementation Method 1
the individual magnetic moments of the nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency
Implementation Method 2
If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or 'tipped', into the x-y plane to produce a net transverse magnetic moment Mxy
Implementation Method 3
The gradient waveforms indicate the positions in k-space at which RF energy should be deposited in order to produce the desired excitation
Data Source
AI summary
A method for producing a spatially and spectrally selective radiofrequency (“RF”) excitation pulse includes establishing a desired spatial RF excitation pattern and establishing a desired spectral RF excitation pattern. The method also includes estimating an RF transmission profile map indicative of the transmission characteristics of an RF coil and determining, from the desired spatial and spectral excitation patterns and the estimated RF transmission profile map, at least one magnetic field gradient waveform indicative of locations in k-space to which RF energy is to be deposited. The method further includes determining, from the established spatial and spectral excitation patterns, the estimated RF transmission profile map, and the determined at least one gradient waveform, at least one RF excitation pulse waveform that will produce the desired spatial and spectral excitation patterns.